Conventional brushless DC motors known include a core type motor shown in FIG. 50 and a coreless type motor as shown in FIG. 51 to FIG. 57.
As shown in FIG. 50, the above core type motor 52 has a stator yoke 53 formed by layering electromagnetic steel plates which are punched out into a certain form, and has a magnetic field coil 54 disposed in a slot section of the stator yoke 53 in the insulated state. A rotor magnet 56 is disposed on a rotor 55. And, a motor section comprises the stator yoke 53 and the rotor 55, and a circuit unit section 57 consists of a circuit element and the like.
The above coreless type motor 60 has a magnetic field coil 59, which is formed in the shape of a ring by a self-fusing line, disposed in an air-core type stator yoke 58 as shown in FIG. 51. The magnetic field coil 59 is formed as shown in, for example, FIG. 52 and FIG. 53. And, a rotor 61 is provided with a rotor magnet 63. And, the stator yoke 58 and the rotor 61 form a motor section, and a circuit unit 76 consists of a circuit element and the like.
In the motors 51 and 60 having the above structures, assembling can be made by fitting from one direction of a motor bearing supporter (boss).
The coreless type motor 60 is known described in, for example, Japanese Patent Application Laid-open Print No. 23754/1989. The brushless DC motor 60 of this type has a rotor 61 consisted of a cup-shaped rotor yoke 62, a ring-shaped rotor magnet 63 adhered to the inner periphery of the rotor yoke, and a shaft 65 fitted to the center of the rotor yoke 62 with a bush 64 as shown in FIG. 54. In the figure, 66 represents a turn table.
As shown in FIG. 55, a stator 67 comprises first and second stator yokes 68, 69 made of a soft magnetic metal plate, and a coil unit 70 disposed between the stator yokes 68, 69. Both the stator yokes 68, 69 have magnetic poles 68a, 69a formed which are extended from the center radially toward outward and bent at right angles, and have rings 71, 72 integrally formed at the center along the bent direction of the magnetic poles 68a, 69a. In the figure, four poles are shown.
The first stator yoke 68 and the second stator yoke 69 of the stator 67 are put together with the coil unit therebetween, the ring 71 and the other ring 72 are joined to be magnetically connected to form a magnetic circuit, keeping a certain gap from the circumferential face of the rotor magnet 63 of the rotor 61, and the magnetic poles 68a, 69a of the first and second stator yokes 68, 69 are alternately disposed in the circumferential direction.
Further, a sleeve 73 is fitted within the ring 71 of the stator 67, and the shaft 65 of the rotor 61 is supported in the sleeve 73 via a bearing 74. To the bottom of the sleeve 73, a base plate 75 is fitted to support the stator 67, and a circuit board 76 is fixed to the base plate 75. In the figure, 77 represents a magnetic pole sensing element to detect a magnetic pole of the rotor.
Besides, as a chucking magnet to adsorb and fix a revolving work, a chucking magnet 79 magnetized as shown in FIG. 57 has been used heretofore.
Furthermore, as brushless DC motors structured using stator yokes which are formed by bending a soft magnetic metal plate, for example, Japanese Patent Application Laid-open Print No. 23754/1989 and Japanese Patent Application Laid-open Print No. 214458/1990 are known. In such motors, magnetic poles are formed in an asymmetrical shape to shift the rotor's static stable position from the position of excitation torque 0, the magnetic poles of one stator yoke have a different shape from those of the other stator yoke, and the position of each stator is shifted to cause magnetic unbalance to shift the rotor's stable position, thereby eliminating a dead point of the motor revolution.
However, the above conventional brushless DC motors have the following structural drawbacks regardless of the core or coreless type.
1) A boss is disposed for a bearing at the center of the motor, a relatively large hole is disposed at the center of the circuit unit to avoid it, this hole spoils a degree of freedom for arrangement and an effective area for circuit parts, and therefore, the shape of the motor body becomes large.
2) As the circuit unit is disposed to be covered by the motor section, when assembling is completed, removal of the circuit unit alone can not be made. And, replacement, modification and adjustment were difficult.
3) The end of the winding of the magnetic field coil is connected to the circuit board by leading around the magnetic field coil to wire, so that a layer short or the like easily takes place due to the breaking of wire or contact of windings during operation, and in particular, the coreless type has a drawback of causing a lot of troubles in wiring and increasing work manhours.
And, in particular, the coreless type motor had the following structural drawbacks.
1) As the stator has a structure to be fitted by mutually putting together the rings of both the stator yokes, precision is required in processing the rings, resulting in increasing costs and causing a lot of troubles in assembling.
2) As the sleeve for supporting the bearing is inserted in the fitted ring section, the number of parts is increased, increasing assembling works.
3) In miniaturizing the motor, magnetic saturation may be caused in the magnetic circuit around the shaft.
4) As the turn table which is put on the rotor is separately formed, the assembling work is increased, and it is difficult to secure the flatness of the turn table.
5) As the base plate is fitted at the bottom of the sleeve passed through the stator, to secure perpendicularity of the sleeve, processing precision of the fitting part of the base plate is required, and sufficient strength cannot be easily obtained. As shown in FIG. 56, the stator 67 can be fixed to the flat base plate 75 by screwing with a screw 78, but a disposing space cannot be provided between the base plate 75 and the stator 67, and it is difficult to dispose the circuit board and the magnetic pole sensing element.
6) And, when assembling, a fitting position of the magnetic pole sensing element has to be determined every time, degrading the precision of positioning and lowering efficiency of assembling work.
7) The chucking magnet 79 for adsorbing and fixing a revolving work is generally magnetized into upper and lower two poles, so that its attraction force is weak, and when the chucking magnet does not have a magnetic substance on the bottom face, leakage flux flows into the stator yoke or rotor yoke, causing a drawback of adversely effecting on the motor performance.
Further, regardless of the core or coreless type motor, there are following drawbacks in the motor characteristics.
1) As the excitation torque generated at the stable point position of the permanent magnet rotor is relatively small, the start up is unstable and can be self-activated only under a very limited condition, and a large start torque cannot be obtained.
2) Since a magnetic balance has to be destroyed, a magnetic pole shape becomes special, and leakage flux is increased, thus the motor efficiency is lowered.
3) As the two stator yokes have different magnetic pole shapes, two press molds are required, and management of parts becomes complicated.
In view of the above, this invention is to solve the above drawbacks and aims to simplify the connection of the coil unit and the circuit unit, and makes it easy to replace the circuit unit alone and adjust even after the assembling of the motor, and reduces the number of fitting parts, facilitates the assembly, reduces manhour, improves the motor performance, and provides an inexpensive brushless DC motor having a high starting torque, providing starting ability in a wide range, having a high motor efficiency and productivity.